Only a very select few in the US military know what it's for, but observers on Earth believe they're putting together the puzzle piece by piece.

Several sources claim quote arms control advocates who say it's clearly the beginning of the "weaponisation of space".

In May, avid skywatcher Ted Molczan studied the X-37B's orbit from his home in Toronto and said its behaviour suggested it was testing sensors for a range of new spy satellites.

Since then, the X-37B been arguably the least-secret secret project on the planet, as fellow backyard astronomers joined in the scrutiny, aided by how-to video guides and apps such as the Simple Satellite Tracker.

That is, they did until July 29, when the shuttle disappeared, causing all kinds of consternation and conspiracy theories about its fate.

It took amateur skywatcher Greg Roberts of Cape Town, South Africa, who noticed that it failed to appear as scheduled above his base on August 14, another five days to find it.

When he did, he noticed it was some 30km higher and on a different trajectory, according to calculations from other colleagues in Rome and Oklahoma.

The X-37B's new track means it takes six days to pass the same spot on Earth, as opposed to its original four-day track.

Mr Molczan believes this may be another small piece to the puzzle about what role the shuttle may play in US military operations.

"This small change of orbit may have been a test of OTV-1's manoeuvring system, or a requirement of whatever payload may be aboard, or both," he said in a release paper about Roberts' X-37B find.

The shuttle has been in orbit now for 124 days. It uses a solar array once in space for power, which theoretically will allow it to stay airborne for up to 270 days.

But the additional presence of large fuel tanks and a rocket motor allows it to change orbit, as evidenced by the latest sudden change of course.

According to the The Register,  this is a key component of its surveillance-related capabilities, along with the fact it can land in a much more versatile fashion than other shuttles.

Using its "cross-range" wings, it can duck off elsewhere once its entered the Earth's atmosphere rather than follow its oribital track to a pre-specified landing pad.

This means the X-37B can get up and down from space in one orbit, as its wings allow it to compensate for the slight turn in the Earth and bend it back to its original launch pad.

The Register says that capability would make it difficult to track, as it would only pass over a region once.

Theoretically, it could drop a spy satellite on one run, then pick it up on the next without the satellite having ever been detected.

Other observers claim the X-37B can carry a payload roughly the size of a medium-sized truck bed, or enough to hold a spy satellite.

According to the Pentagon, a second X-37B is under construction, so expect the guessing game to continue for some time about what the US military is really up to in space.

Until now, all that remains known about the X-37B is that is it has at least one trick - the ability to hide from skywatchers for two weeks.